allow LED initialization without STATUS_LED_BOOT

For current U-Boot to initialize status LEDs via status_led_init(), it
is required to have both CONFIG_STATUS_LED and STATUS_LED_BOOT defined.
This may be a particular concern with GPIO LEDs, where __led_init() is
required to correctly set up the GPIO (gpio_request and
gpio_direction_output). Without STATUS_LED_BOOT the initialization isn't
called, which could leave the user with a non-functional "led" command -
due to the fact that the LED routines in gpio_led.c use gpio_set_value()
just fine, but the GPIO never got set up properly in the first place.

I think having CONFIG_STATUS_LED is sufficient to justify a
corresponding call to status_led_init(), even with no STATUS_LED_BOOT
defined. To do so, common/board_r.c needs call that routine, so it now
is exposed via status_led.h.

Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>
[trini: Add dummy __led_init to pca9551_led.c]
Signed-off-by: Tom Rini <trini@konsulko.com>